Corporations are an easy scapegoat that also fits a political ideology on the left.
It's also a complete fallacy to claim that people cannot see through things. People are not stupid they know that it is their diet that makes them fat. People deserve at least this credit and respect.
Alcohol and cigarettes sales to children are obviously completely different and a very poor analogy.
So... Following the logic, either we accept that people are free to make their own decisions, that parents are responsible for their children, or we take children away from parents because the state knows best.
Personally I think we should do the maximum we can to teach good diets at schools, and perhaps provide classes for adults, and to label products then people can do whatever they want but live with the consequences.
But I think the likely outcome (as we start to see in some countries), which must be politically acceptable and cheap, will be similar to the path taken for cigarettes: PR campaigns and tax. This is a way to let people do what they want while nudging them, and to let them pay for the consequences (healthcare where it is financed through tax).
When it comes to children parents won't be blamed (although they should be) because this is not good to win votes. It will be easier to continue blaming ads and corporations, and to ban things.